1. Field of the Invention
This invention relates to image-forming materials, specifically to color-providing compounds which, in the presence of silver ions and/or a soluble silver complex, undergo a cleavage reaction to liberate two or more color-providing moieties.
2. Description of the Related Art
U.S. Pat. No. 3,719,489 discloses silver ion assisted cleavage reactions useful in photographic systems. As disclosed therein, photographically inert compounds are capable of undergoing cleavage in the presence of silver ions made available imagewise during processing of a silver halide emulsion to liberate a reagent, such as, a photographically active reagent or a dye in an imagewise distribution corresponding to that of said silver ions. In one embodiment disclosed therein, color images are produced by using as the photographically inert compounds, color providing compounds which are substantially non-diffusible in the photographic processing composition but capable of undergoing cleavage in the presence of the imagewise distribution of silver ions and/or soluble silver complex made available in the undeveloped and partially developed areas of a silver halide emulsion as a function of development to liberate a more mobile and diffusible color-providing moiety in an imagewise distribution corresponding to the imagewise distribution of said ions and/or said complex. The subsequent formation of a color image is the result of the differential in diffusibility between the parent compound and liberated color-providing moiety whereby the imagewise distribution of the more diffusible color-providing moiety released in the undeveloped and partially developed areas is free to transfer.
Color-providing compounds useful in the above process form the subject matter of U.S. Pat. No. 4,098,783, a continuation in part of said U.S. Pat. No. 3,719,489. The color-providing compounds disclosed therein may comprise one or more dye radicals and one or more 1,3-sulfur-nitrogen moieties. For example, they may comprise one complete dye or dye intermediate and one cyclic 1,3-sulfur-nitrogen moiety. Alternatively, the color-providing compounds may comprise two or more cyclic moieties for each dye radical or dye intermediate and vice versa. Particularly useful dye-providing compounds disclosed therein comprise a dye containing from 1 to 4 and preferably 1 or 2 cyclic 1,3-sulfur-nitrogen groups and may be represented by the formula EQU D-[(L).sub.m-1 -Y].sub.n (A)
wherein D represents a dye radical, i.e., the radical of an organic dye possessing at least one carbon atom, L is a divalent organic linking group containing at least one carbon atom, m is a positive integer 1 or 2, n is a positive integer from 1 to 4, and Y is a cyclic 1,3-sulfur-nitrogen group.
U.S. Pat. No. 4,468,448 discloses a different class of 1,3-sulfur-nitrogen compounds which, rather than relying on the differential in diffusibility between the colored parent compound and the liberated dye to form the color image, as in the aforementioned U.S. Pat. No. 3,719,489, utilize the ability of 1,3-sulfur-nitrogen compounds to undergo silver ion assisted cleavage to provide an imagewise distribution of a colored image dye from a substantially colorless precursor of a preformed image dye. This is accomplished by employing a 1,3-sulfur-nitrogen group to maintain said precursor in its substantially colorless form until said 1,3-sulfur-nitrogen group undergoes cleavage. The color image may be formed by using the imagewise cleavage of the 1,3-sulfur-nitrogen group to provide the image dye directly, or the imagewise cleavage of the 1,3-sulfur-nitrogen group may be used to activate a subsequent reaction or series of reactions which in turn provide the image dye.
Thermally developable black and white as well as color photosensitive materials, whose development is effected by heating, are well known. Among the systems designed to give color images are those wherein a diffusible dye is released as a result of the heat development of an organic silver salt and transferred to the image-receiving layer, whereby a color image is obtained.
Japanese Kokai 59-180548 having a Laid-Open date of Oct. 13, 1984 discloses a heat-developable silver halide photosensitive imaging system wherein the dye-providing material contains a heterocyclic ring containing a nitrogen atom and a sulfur or selenium atom which heterocyclic ring is subject to cleavage in the presence of silver ions to release a diffusible dye. An example of a suitable dye-providing material is a thiazolidine dye such as disclosed in the aforementioned U.S. Pat. No. 4,098,783. The process involves imagewise exposing the photosensitive system to light and subsequently or simultaneously heating the photosensitive system under a substantially water-free condition, in the presence of a base or base precursor, whereby an oxidation-reduction reaction between the exposed photosensitive silver halide and a reducing agent occurs. In the exposed areas a negative silver image is formed. In the unexposed areas, the silver ion, present in inverse proportion to the silver image, causes the heterocyclic ring of the dye-providing material to be cleaved releasing a diffusible dye. The diffusible dye is then transferred to an image-receiving layer whereby a positive dye image is formed.
While the differential in diffusibility between the parent compound and the liberated color-providing moiety, disclosed in the aforementioned U.S. Pat. No. 3,719,489, is useful in obtaining a color image, under some conditions a small amount of the parent compound may also transfer. Thus, in color diffusion transfer film products wherein the parent compound comprising a colored image dye-providing moiety is itself colored, non-imagewise diffusion during processing of even a minimal amount of the parent compound to a receptive layer of the film unit can adversely affect the quality of the image, particularly in the Dmin, i.e., highlight, areas of the image. This has been found to be a particularly acute problem in thermally developed silver halide photographic systems.
One way to lessen the diffusion of uncleaved dye-providing material is to add additional ballasting groups and/or to increase the size of the ballast groups. However, this is not very efficient since it requires a larger weight of uncleaved dye-providing material for the same amount of dye released.
It has been now been found that by using additional dye providing radicals as ballast groups, one can decrease diffusion of the uncleaved parent compound to the receptive layer of the film unit while increasing the image-forming efficiency of the color-providing materials, i.e., releasing more dye-providing moieties per molecule of uncleaved color-providing material.